U.S. Pat. No. 5,503,012 to Rabizadeh issued on Apr. 2, 1996 discloses a tire pressure monitoring device that includes a ball disposed within a tube which has an opening, at one end, in communication with the interior of the tire. The tube has a transparent window along its length to allow the position of the tire pressure ball to be visualized. The transparent window has graduations, calibrated to allow the tire pressure to be determined by the position of the ball. Nevertheless, reading of the tire pressure among the indicia is not easy, and an unskilled user may report a wrong tire pressure. In addition, the indicia on the transparent window tend to wear out after a term of use. In addition, the compression spring for attaching the ball is more or less rotated during movement of the ball due to the helical configuration of the compression spring. Namely, unexpected rotational movement of the compression occurs and thus affects the pressure indication accuracy.
U.S. Pat. No. 4,919,600 to Yang issued on Apr. 24, 1990 discloses a tire pump with a pressure gauge including a cylinder having a piston head slidably provided in the cylinder. A pressure measuring device with a pressure indicating element is disposed at a front end of the cylinder so that the pressure indicating element is readable through a peep hole in the cylinder. The pressure measuring device interconnects an outlet of the cylinder. A bypass tube with a check valve is disposed beside the pressure measuring device so that air pressurized by the piston head flows out from the outlet through the bypass tube. The pressurized air also flows through air holes in order that the pressure within the pumped tire can be read from the pressure indicating element of the pressure measuring device through the peep hole. Again, the compression spring for attaching the pressure indicating element is more or less rotated during movement of the pressure indicating element due to the helical configuration of the compression spring. Unexpected rotational movement of the compression occurs and thus affects the pressure indication accuracy. The pressure indicating element includes a scale attached to a piston, wherein the scale tends to break during use, and the manufacture cost for such a pressure indicating element is relatively high.
FIG. 1 of the drawings illustrates another conventional arrangement of a pressure indicator 50 that is square in section and is slidably received in a chamber 51 with square section. The pressure indicator 50 has pressure indicia marked on an outer periphery thereof for indicating the tire pressure. However, the friction between the pressure indicator 50 and the chamber 51 is so large such that the pressure indicated by the pressure indicator 50 is not precise.
The present invention intends to provide a pressure gauge that may provide accurate tire pressure indication.